1. Field of the Invention
The present invention generally relates to a method of storing information in a recording medium, and more particularly, to a method of writing multi-level data to a recording medium such as an optical disk. The present invention also relates to an information storage apparatus in which the method of storing information is used and to a recording medium in which information has been stored using the method.
2. Description of the Related Art
The recording density of an optical recording medium (optical disk) such as a Compact Disk (CD) and a Digital Versatile Disk (DVD) is desired to further increase. One approach to the further increase in the recording density of an optical recording medium is to improve the performance of an optical pickup that write and read data to an optical recording medium. Another approach is to write and read multi-level data to an optical recording medium.
In the case of phase change type optical information storage apparatus, bi-level recording in which whether a recording mark is formed in a recording unit (cell) expresses one bit is conventionally used. Multi-level recording is a technique in which a recording mark is formed in a recording unit but the length of the recording mark is controlled step-wise thereby to express multiple bits. That is, multi-level data is written by changing step-wise the ratio of amorphous mark to crystalline base in a cell, and read by detecting the intensity of reflective light from the cell.
However, since the recording properties of each optical recording medium and those of each optical information storage apparatus may vary due to individual difference and environmental change, the write power and erase power of a laser beam and time period (write pulse width) in which the laser beam of write power is emitted need to be determined taking the individual difference and the environmental change into account. For example, various techniques for determining write power are proposed.
For example, Japanese Patent Laid-Open Application No. 10-134353 discloses a technique in which test writing is repeated until ideal signal wave is obtained in order to surely reproduce multi-level data.
Japanese Patent Laid-Open Application No. 2003-91822 discloses a technique in which test writing is performed with write power being gradually changed and a write power that saturates reflective light quantity is determined as the optimal write power. This technique makes change in thermal diffusion easily detectible caused by excessive write power.
However, the conventional technique described above has the following problem. The technique disclosed in above Japanese Patent Laid-Open Application No. 10-134353 performs adjustment by loop processing including the steps of: writing and reading test data, comparing signal obtained by reading the written test data with the ideal signal waveform, determining whether the adjustment has converged (if converged, the process is terminated), and adjusting condition of laser emission. However, this technique requires the writing of test data many times and huge number of arithmetic operations, resulting in long time for test recording.
In the case of multi-level data recording system that forms a recording mark, the length of which is changed step-wise, in a recording cell, each value of the multi-level data is written in a recording cell of fixed length on a recording track 51 as shown in FIG. 25. A recording mark is formed in each recording cell 52, and the level of reproduced signal from the recording mark 53 depends on the length of the recording mark 53. When multi-level data is read, the reproduced signal from the recording mark 53 is sampled at predetermined frequency (for example, at the timing in which a reproduction light spot 54 comes to the center of the recording cell 52), and the value of the multi-level data is discriminated based on the reflective light intensity of the sampled reproduced signal. Since the spot size of the reproduction light spot 54 is longer than the length of the recording cell 52 in the circumference directions of the optical disk, interference between data written in adjacent recording cell occurs. Generally, the interference is taken into consideration when discriminating the multi-level data.
In the case of eight-level data recording system, for example, 83=512 combinations need to be taken into account. Time required for arithmetic operation is not negligible. Additionally, because the above conventional technique needs to be repeated to increase accuracy, long processing time is not avoidable.
Above Japanese Patent Laid-Open Application No. 2003-91822 discloses a recording technique to determine the optimal recording condition (write power and write pulse width) easily without involving the steps of the previous conventional technique. According to this recording technique, test recording is performed with write power being varied, and write power with which the reflective light intensity saturates is determined as the optimal write power. However, this recording technique uses conventional write waveform to write multi-level data, and the following problem still remains unsolved.
The write waveform indicates how the power of laser beam is changed in time to write a single value of multi-level data. As shown in FIG. 26, a conventional write waveform includes a top pulse in which the laser beam of write power Pw is emitted for pulse width Ton, an off pulse in which the laser beam of bias power Pb is emitted for pulse width Toff, and an erase pulse in which the laser beam of erase power Pe is emitted for remaining time period of the recording cell. When such write waveform is used to write different values of multi-level data, the same top pulse having the same write power Pw and the same pulse width Ton is used, but different off pulses only the pulse width of which are different are employed. That is, the length of a recording mark is determined only by the pulse width Toff of the off pulse in which the recording layer heated by the top pulse is rapidly cooled.
FIG. 27 shows relation between the write power Pw and multi-level data error rate (SER) . The SER is the rate of the number of multi-level symbol errors divided by the total number of symbols. Low SER means low probability of erroneous detection of multi-level data, and the consequently accurate discrimination of multi-level data. As shown in FIG. 27, the SER draws a curve that becomes minimum at the optimal write power P0, but is not symmetric at both sides thereof. FIG. 27 shows that low power recording (Pw<P0) results in the rapid degradation (increase) of SER compared to high power recording (Pw>P0). The cause of the rapid degradation of SER is considered to be that the low power recording cannot raise the temperature of the recording layer enough, and as a result, cannot form a recording mark of desired area.
If the optimal write power P0 shifts to the high power side due to the diversity of individual product and the environmental change, but the same write power is used for recording, the SER may be increased beyond the limit in which the multi-level data cannot be read. That is, margin (power margin) at the low power side is small. The small power margin results in the following problems.
The substrate of an optical disk made of polycarbonate is not flat as a glass substrate, and may be bent and undulant. For example, if the optical disk is bent in the direction from the inner circumference to the outer circumference, the reflective light from the optical disk enters the optical pickup forming different incident angles, and as a result, the level of reproduction signal changes in dependence of the position of the optical pickup in the radial direction. If the power margin is small, SER becomes sensitive to the change in the reproduction signal. To solve this problem, various conditions such as write power needs to be determined in dependence on the position in the radial direction thereby to keep SER low. As a result, recording and reproduction processing takes long time. Additionally, some areas for test recording need to be provided in some radial positions in order to determine the various conditions such as the write power. However, the areas for test recording reduces user region, resulting in the decrease in recording capacity of the optical disk.
Furthermore, if the recording track is wound, the laser beam is defocused in a heavily wound region. The writing of data using defocused laser beam is substantially equivalent to low power recording. FIG. 27 suggests that SER would be degraded in a region in which focus servo fails locally due to great winding. As a result, if the power margin is small in the low power side, multi-level data written on some optical disks may be unable to reproduce due to divergence of each optical disk substrate.
Recently, recordable CD and DVD that can write data at higher speed than the standard speed have been developed. An example of information storage methods applied to such recordable CD and DVD includes a method of adjusting reference clock cycle of recording signal in dependence on the radial position while the disk rotative speed maintained constant in order to realize high speed CLV (Constant Linear Velocity) recording. However, if the above method is applied to the multi-level recording described above, since the recording sensitivity of the optical disk is not linear to the recording linear velocity, the recording mark formed at the inner circumference of the optical disk may be different from the recording mark formed at the outer circumference of the optical disk.
Especially, at the outer circumference of the optical disk, the recording linear speed becomes high resulting in low recording sensitivity. In the case in which low multi-level data values (1-3) is recorded, a recording mark having sufficient size can not be formed. The corresponding relation between the multi-level data and the reflective light intensity may deviate from that of standard recording linear velocity recording. As a result, multi-level discrimination level and waveform equalization factor may need to be learned for each recording linear velocity. In reproduction operation, the multi-level discrimination level and the waveform equalization factor may need to be retrieved corresponding to each linear velocity, but this process may take too long time.